Cage-shaped cyclopentanoic dianhydride, method for production thereof, and polyimide

ABSTRACT

A cage 1,2,3,4-cyclopentanetetracarboxylic acid (1,3:2,4)-dianhydride compound represented by formula [1], and a polyimide obtained by condensing the compound with a diamine. With the compound, it is possible to provide a polyimide which shows no absorption in the ultraviolet region and is highly transparent to light, has high insulating properties, has improved heat resistance and processability, and has excellent solubility in organic solvents. 
                         
(In formula [1], R 1  and R 2  each independently represents a hydrogen atom, a halogen atom, or a C 1-10  alkyl.)

This application is a Divisional of co-pending application Ser. No.13/263,456, filed on Jan. 9, 2012, which is the National Phase ofInternational Application No. PCT/JP2010/056222 filed on Apr. 6, 2010,which claims priority to Japanese Patent Application No. 2009-095953filed Apr. 10, 2009, all of which are hereby expressly incorporated byreference into the present application.

TECHNICAL FIELD

The present invention relates to a cage-shaped cyclopentanoicdianhydride, a method for production thereof, and a polyimide. Moreparticularly, the present invention relates to a cage-shapedcyclopentanetetracarboxylic dianhydride as a monomer for the polyimidewhich is suitable for use as an optical material and the like and alsoto a method for production of the compound.

BACKGROUND ART

On account of their good mechanical strength, heat resistance,insulating properties, and solvent resistance, polyimides are in generaluse as an electronic material, such as protective material, insulatingmaterial, and color filter, for liquid crystal display units andsemiconductors. They are also expected to find new uses as the materialof optical waveguide for optical communications and as the material ofsubstrates for mobile phones.

The recent remarkable development in these fields has come to requirematerials with more sophisticated properties than before. In otherwords, the polyimide used in these fields needs not only good heatresistance and solvent resistance but also many other properties, suchas transparency, for individual applications.

The conventional polyimide in general use is a total aromatic polyimidewhich is obtained by polycondensation reaction between an aromatictetracarboxylic dianhydride and an aromatic diamine. Unfortunately,because of its dark amber color, the total aromatic polyimide posesproblems in application areas where high transparency is necessary. Inpractice, the total aromatic polyimide is insoluble in organic solvent,which makes it necessary to form its film from polyamic acid as itsprecursor by dehydrocyclization with heating.

One way of achieving good transparency is by the polycondensationreaction between an alicyclic tetracarboxylic dianhydride and anaromatic diamine, which yields a polyimide precursor, and the ensuingimidization of the precursor. This process is known to give a highlytransparent polyimide with comparatively less discoloration (See PatentDocuments 1 and 2).

Unfortunately, the polyamic acid and polyimide formed from anunsubstituted alicyclic tetracarboxylic dianhydride are hardly solublein ordinary organic solvents and only soluble in high-boiling polarsolvents. This necessitates heating at high temperatures for solventremoval in the film-forming process. Heating adversely affects any otherorganic materials constituting the organic EL element.

There has recently been reported a research on making the gas barrierfilm for the organic electroluminescence (EL) element from a polyimidepolymerized from 1,2,3,4-cyclopentanetetracarboxylicacid-1:2,3:4-dianhydride (CPDA for short hereinafter) (See PatentDocument 3).

Unfortunately, this polyimide still has room for improvement in heatresistance because of its low degree of polymerization and is notnecessarily satisfactory in solubility in organic solvents.

PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: JP-B H02-24294-   Patent Document 2: JP-A S58-208322-   Patent Document 3: JP-A 2006-232960

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

The present invention was completed in view of the foregoing. It is anobject of the present invention to provide an alicyclic tetracarboxylicdianhydride, a method for efficient and economical production thereof,and a polyimide formed therefrom, the compound being a monomer for thepolyimide which excels in transparency without absorption in the UVregion, insulating properties, heat resistance, processability, andsolubility in organic solvents.

Means for Solving the Problems

The present inventors carried out extensive researches to achieve theforegoing object by paying attention to increasing the linearity of themain chain of the polyimide structure, thereby raising the degree ofpolymerization. As the result, they found a cage-shapedcyclopentanetetracarboxylic dianhydride as the monomer for the polyimidewhich, because of its good linearity, has a high degree ofpolymerization and excels in heat resistance and solubility in organicsolvents. This finding led to the present invention.

The present invention covers the following.

1. A cage-shaped 1,2,3,4-cyclopentanetetracarboxylicacid-1,3:2,4-dianhydride represented by the formula [1].

(where R¹ and R² independently denote a hydrogen atom, halogen atom, orC₁₋₁₀ alkyl group.)2. The cage-shaped 1,2,3,4-cyclopentanetetracarboxylicacid-1,3:2,4-dianhydride as defined in Paragraph 1 above, wherein R¹ andR² each denotes a hydrogen atom.3. A trans,trans,trans-1,2,3,4-cyclopentanetetracarboxylic acidrepresented by the formula [2].

(where R¹ and R² independently denote a hydrogen atom, halogen atom, orC₁₋₁₀ alkyl group.)4. A trans,trans,trans-1,2,3,4-cyclopentanetetracarboxylic tetraalkylester represented by the formula [3].

(where R¹ and R² independently denote a hydrogen atom, halogen atom, orC₁₋₁₀ alkyl group, and R³ denotes a C₁₋₁₀ alkyl group.)5. A method including

a first step of reacting cis,cis,cis-1,2,3,4-cyclopentanetetracarboxylicacid-1,2:3,4-dianhydride represented by the formula [4].

(where R¹ and R² independently denote a hydrogen atom, halogen atom, orC₁₋₁₀ alkyl group.)with an alcohol represented by the formula [5]R³OH  [5](where R³ denotes a C₁₋₁₀ alkyl group.)in the presence of an acid catalyst, thereby givingcis,cis,cis-1,2,3,4-cyclopentanetetracarboxylic tetraalkyl esterrepresented by the formula [6]

(where R¹, R², and R³ are defined as above.)

a second step of isomerizing in the presence of a base catalyst thecompound represented by the formula [6] above, which was obtained in thefirst step, thereby givingtrans,trans,trans-1,2,3,4-cyclopentanetetracarboxylic tetraalkyl esterrepresented by the formula [3]

(where R¹, R², and R³ are defined as above.)

a third step of decomposing with the help of an organic acid thecompound represented by the formula [3] above, which was obtained in thesecond step, thereby givingtrans,trans,trans-1,2,3,4-cyclopentanetetracarboxylic acid representedby the formula [2]

(where R¹ and R² are defined as above.)and

a fourth step of dehydrating the compound represented by the formula [2]above, which was obtained in the third step, thereby giving acage-shaped 1,2,3,4-cyclopentanetetracarboxylic acid-1,3:2,4-dianhydriderepresented by the formula [1].

(where R¹ and R² are defined as above.)6. The production method as defined in Paragraph 5, wherein the acidcatalyst used in the first step is sulfuric acid.7. The production method as defined in Paragraph 5, wherein the basecatalyst used in the second step is metal alcoholate.8. The production method as defined in Paragraph 7, wherein the basecatalyst is potassium t-butoxide.9. The production method as defined in Paragraph 5, wherein theisomerization in the second step is accomplished at 0 to 200° C.10. The production method as defined in Paragraph 5, wherein the organicacid used in the third step is formic acid.11. The production method as defined in Paragraph 5, wherein thedecomposition with the help of organic acid is accomplished at 0 to 200°C.12. The production method as defined in Paragraph 5, wherein thedehydration in the fourth step is accomplished with the help of anorganic acid anhydride.13. A polyamic acid which contains the repeating unit represented by theformula [7] below in an amount of at least 10 mol %.

(where A denotes a tetravalent organic group represented by the formula[8] and B denotes a divalent organic group and n is an integer.)

(where R¹ and R² independently denote a hydrogen atom, halogen atom, orC₁₋₁₀ alkyl group; and a1 to a4 denote the positions for bonding withthe carbon atom of the carbonyl group in the formula [7], provided thatbonding with the carboxyl group does not take place simultaneously at a1and a3 and bonding with the carboxyl group does not take placesimultaneously at a2 and a4.)14. The polyamic acid as defined in Paragraph 13 wherein the R¹ and R²each is a hydrogen atom or methyl group.15. The polyamic acid as defined in Paragraph 13 wherein the B is adivalent organic group derived from an alicyclic diamine or aliphaticdiamine.16. The polyamic acid as defined in Paragraph 13 or 14, wherein the B isat least one species selected from the divalent organic groupsrepresented by the formulas [9] to [12].

(where R⁴ to R¹¹ independently denote a hydrogen atom, halogen atom, orC₁₋₁₀ alkyl group, and m is an integer of 1 to 10.)17. The polyamic acid as defined in Paragraph 16, wherein the B isrepresented by the formula [13].

18. The polyamic acid as defined in Paragraph 16, wherein the B isrepresented by the formula [14].

19. The polyamic acid as defined in Paragraph 16, wherein the B isrepresented by the formula [15].

20. The polyamic acid as defined in Paragraph 16, wherein the B isrepresented by the formula [16].

21. A polyimide which is obtained by cyclodehydration from any one ofthe polyamic acids defined in Paragraphs 13 to 20.

Advantageous Effect of the Invention

According to the present invention, there are provided a cage-shaped1,2,3,4-cyclopentanetetracarboxylic acid-1,3:2,4-dianhydride (“cage”CPDA for short hereinafter) and a method for efficient productionthereof, the compound giving a polyimide which has a high degree ofpolymerization and exhibits good heat resistance and high solubility inorganic solvents. Owing to its improved heat resistance and good opticaltransparency without absorption in the UV region, the resultingpolyimide is expected to find use as a protecting and insulatingmaterial for electronic devices such as liquid crystal displays andsemiconductors and also as waveguide for optical communications.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an X-ray ORTEP diagram of a single crystal of “cage” CPDAwhich was obtained in Example 4.

FIG. 2 is a ¹H-NMR spectrum of “cage” CPDA-1,3-BAPB polyimide which wasobtained in Example 5.

FIG. 3 is a ¹H-NMR spectrum of “cage” CPDA-1,3-BAPB polyimide which wasobtained in Example 6.

FIG. 4 is a ¹H-NMR spectrum of “cage” CPDA-DPP polyimide which wasobtained in Example 7.

FIG. 5 is a ¹H-NMR spectrum of “cage” CPDA-p-PDA polyimide which wasobtained in Example 8.

FIG. 6 is a ¹H-NMR spectrum of “cage” CPDA-DDE polyimide which wasobtained in Example 9.

EMBODIMENT FOR CARRYING OUT THE INVENTION

A detailed description of the invention will follow, in which suchsymbols as n, i, s, t, and c stand for normal, iso, secondary, tertiary,and cyclo, respectively.

In the foregoing formulas, the halogen atom includes fluorine, chlorine,bromine, and iodine atoms.

The C₁₋₁₀ alkyl group includes linear, branched, and cyclic ones, asexemplified by methyl, ethyl, n-propyl, i-propyl, c-propyl, n-butyl,i-butyl, s-butyl, t-butyl, c-butyl, n-pentyl, 1-methyl-n-butyl,2-methyl-n-butyl, 3-methyl-n-butyl, 1,1-dimethyl-n-propyl, c-pentyl,2-methyl-c-butyl, n-hexyl, 1-methyl-n-pentyl, 2-methyl-n-pentyl,1,1-dimethyl-n-butyl, 1-ethyl-n-butyl, 1,1,2-trimethyl-n-propyl,c-hexyl, 1-methyl-c-pentyl, 1-ethyl-c-butyl, 1,2-dimethyl-c-butyl,n-heptyl, n-octyl, n-nonyl, and n-decyl.

The “cage” CPDA according to the present invention can be produced byfour steps shown in the following scheme.

A first step of reacting cis,cis,cis-1,2,3,4-cyclopentanetetracarboxylicacid-1,3:2,4-dianhydride (cis,cis,cis-CPDA for short hereinafter) withan alcohol in the presence of an acid catalyst, thereby givingcis,cis,cis-1,2,3,4-cyclopentanetetracarboxylic tetraalkyl ester(cis,cis,cis-TACP for short hereinafter); a second step of isomerizingin the presence of a base catalyst the cis,cis,cis-TACP, thereby givingtrans,trans,trans-1,2,3,4-cyclopentanetetracarboxylic tetraalkyl ester(trans,trans,trans-TACP for short hereinafter); a third step ofdecomposing the trans,trans,trans-TACP, thereby givingtrans,trans,trans-1,2,3,4-cyclopentanetetracarboxylic acid(trans,trans,trans-CPTC for short hereinafter); and a fourth step ofdehydrating the trans,trans,trans-CPTC, thereby giving the “cage” CPDA.

(where R¹ to R³ are defined as above.)

Incidentally, the cis,cis,cis-CPDA as the raw material in the first stepmay be synthesized by the process shown in the following scheme.

That is, Diels-Alder reaction between cyclopentadiene (CPD) and maleicanhydride, which gives 5-norbornene-2,3-dicarboxylic anhydride (NDA).Then, the oxidation of NDA for conversion into1,2,3,4-cyclopentanetetracarboxylic acid (CPTC). Finally, thedehydration of CPTC to give the desired cis,cis,cis-CPDA.

(where R¹ and R² are defined as above.)

Typical examples of the CPD include cyclopentadiene,1-methyl-2,4-cyclopentadiene, 1-ethyl-2,4-cyclopentadiene,1-n-propyl-2,4-cyclopentadiene, 1-n-butyl-2,4-cyclopentadiene,1-n-octyl-2,4-cyclopentadiene, 1-n-nonyl-2,4-cyclopentadiene,1-n-decyl-2,4-cyclopentadiene, 1,1-dimethyl-2,4-cyclopentadiene,1,1-diethyl-2,4-cyclopentadiene, 1,1-di(n-decyl)-2,4-cyclopentadiene,1-fluoro-2,4-cyclopentadiene, 1,1-difluoro-2,4-cyclopentadiene,1-chloro-2,4-cyclopentadiene, 1,1-dichloro-2,4-cyclopentadiene,1-bromo-2,4-cyclopentadiene, and 1,1-dibromo-2,4-cyclopentadiene.

Incidentally, the cis,cis,cis-CPDA to be produced from cyclopentadieneis commercially available, and any commercial product may be used assuch.

[1] The First Step for Esterification Reaction

The esterification employs any one of such C₁₋₁₀ alkyl alcohols asmethanol, ethanol, n-propanol, i-propanol, n-octanol, and n-decanol. Ofthese alcohols, methanol is economically preferable. It should be usedin an amount (by weight) 2 to 30 times, preferably 3 to 10 times, thereactant.

The esterification also employs an acid catalyst selected from inorganicacids, such as hydrochloric acid and sulfuric acid, solid acids, such asheteropoly acid and cation exchange resin. Of these acids, sulfuric acidis preferable. It should be used in an amount of 0.1 to 20 wt %,preferably 1 to 10 wt %, of the reactant.

The esterification should be carried out at 20 to 200° C., preferably 50to 150° C., in the vicinity of the boiling point of the alcohol.

The progress of esterification can be checked by gas chromatography.When it is found that the reactant has disappeared after esterificationwith the help of sulfuric acid as an acid catalyst, the reaction productis condensed and the resulting oily condensate is extracted with ethylacetate and water. The thus obtained organic layer is washed with waterand dried to give the desired cis,cis,cis-TACP.

[2] The Second Step for Isomerization

The isomerization employs any one of such base catalysts as alkali metalor alkaline earth metal in the form of alcoholate, carbonate, hydroxide,or oxide. Of these compounds, alcoholate is preferable.

The alkali metal includes lithium, sodium, and potassium, and thealkaline earth metal includes magnesium, calcium, and barium.

Preferable among alcoholates are sodium methoxide, sodium ethoxide,sodium t-butoxide, potassium methoxide, potassium ethoxide, andpotassium t-butoxide. Particularly preferable among them are sodiummethoxide and potassium t-butoxide.

The base catalyst should be used in an amount of 0.1 to 50 mol %,preferably 0.5 to 20 mol %, of the reactant.

The solvent for isomerization should preferably be an alcohol,particularly a lower alcohol such as methanol, ethanol, n-propanol, andi-propanol. Of these alcohols, methanol is preferable. It should be usedin an amount (by weight) 3 to 30 times, preferably 5 to 10 times, thereactant.

The reaction temperature for isomerization should preferably be 0 to200° C., particularly 20 to 150° C.

After the completion of isomerization, the reaction product is condensedand the resulting residue is extracted with ethyl acetate and water. Theresulting extract is acidified with 35% hydrochloric acid for separationof the organic layer. The thus obtained organic layer is condensed togive crude trans,trans,trans-TACP. This crude product is purified bysilica gel chromatography to give pure trans,trans,trans-TACP.

[3] The Third Step for Acid Decomposition

The third step employs an acid selected from inorganic acids, such ashydrochloric acid, sulfuric acid, and phosphoric acid, fatty acids, suchas formic acid, acetic acid, and propionic acid, and sulfonic acids,such as methanesulfonic acid, ethansulfonic acid, andtrifluoromethanesulfonic acid. Of these acids, formic acid is preferablebecause it is easy to use. It should be used in an amount more than 4mol equivalents for the reactant.

Formic acid should be used in an excess amount (10 to 100 molequivalent) because formic ester that occurs as a by-product isdistilled away together with formic acid to promote reactions.

The above-mentioned acid should preferably be used in combination withbenzenesulfonic acid or p-toluenesulfonic acid, with the latter beingmore desirable. The amount of such additional acid should be 0.1 to 10wt %, preferably 0.5 to 5 wt %, of the reactant.

This reaction should be continued until the reactant disappears (asindicated by the ¹H-NMR spectrum), with the acid ester as a by-productdistilled away.

The reaction temperature should preferably be 0 to 200° C.

After the completion of reaction, the reaction liquid is condensed andthe condensate is dissolved with heating in acetonitrile added thereto.The resulting solution is slightly condensed and then ice-cooled forcrystallization. The crystals are filtered off and washed with ethylacetate and finally vacuum-dried to give the desiredtrans,trans,trans-CPTC.

Residual p-toluenesulfonic acid may be removed as follow, if it shouldexist. The product is dissolved in ethyl acetate and a small amount ofwater added thereto. The organic layer is condensed and the residue isdissolved with heating in acetonitrile added thereto. The resultingsolution is slightly condensed and ice-cooled for crystallization. Thecrystals are filtered off, washed with ethyl acetate, and vacuum-dried.

[4] The Fourth Step for Dehydration

This step employs a dehydrating agent, such as aliphatic carboxylic acidanhydride, 1,3-dicyclohexylcarbodiimide (DCC), and2-chloro-1,3-dimethylimidazolium chloride (DMC). The first one, which istypically exemplified by acetic anhydride, is preferable because of itslow price. It should be used in an amount of 2 to 50 equivalents,preferably 2 to 10 equivalents, for the reactant.

This step may employ the dehydrating agent in an excess amount as asolvent or employ any organic solvent not involved directly with thereaction.

Examples of such an organic solvent include aromatic hydrocarbons, suchas toluene and xylene, halogenated hydrocarbons, such as1,2-dichloroethane and 1,2-dichloropropane, and 1,4-dioxane. It shouldbe used in an amount (by weight) 1 to 20 times, preferably 1 to 10times, the reactant.

The reaction temperature for this step is usually 50 to 200° C.,preferably 60 to 150° C., in the vicinity of the boiling point of thedehydrating agent or solvent.

The reaction time for this step is 0.1 to 10 hours, preferably 0.2 to 5hours, which varies depending on the reaction temperature.

After the completion of reaction, the reaction product is freed of thedehydrating agent (together with the solvent if necessary) bydistillation. In this way there is obtained “cage” CPDA in high purity.It may be further purified by recrystallization according to need.

If the third step employs formic acid, the third and fourth steps may becombined into one step. In this case, the reaction mixture obtained inthe third step undergoes dehydration in such a way that formic acid andacetic acid (that occurs as a by-product if acetic anhydride is used asthe dehydrating agent) are distilled away together with the optionalorganic solvent. In this way the desired “cage” CPDA is obtained with ahigh degree of conversion.

The above-mentioned reactions may be carried out batchwise orcontinuously, with or without compression.

The “cage” CPDA of the present invention, which has been obtained asmentioned above, may undergo polycondensation with a diamine to give apolyamic acid represented by the formula [7] below, which subsequentlyundergoes dehydrocyclization with heating or catalyst, so that it isconverted into its corresponding polyimide.

(where A denotes a tetravalent organic group represented by the formula[8] and B denotes a divalent organic group and n is an integer.)

(where R¹ and R² are defined as above; and a1 to a4 denote the positionsfor bonding in the formula [7], provided that bonding with the carboxylgroup does not take place simultaneously at a1 and a3 and bonding withthe carboxyl group does not take place simultaneously at a2 and a4.)

The cyclopentane skeleton represented by the formula [8] has thetrans-trans-trans conformation indicated by a1 to a4.

In the formula [8] above, R¹ and R² independently denote a hydrogenatom, halogen atom, and C₁₋₁₀ alkyl group, with a hydrogen atom and amethyl group being preferable. In other words, the “cage” CPDA as thedesirable starting material is any of cage-shaped1,2,3-4-cyclopentanetetracarboxylic acid-1,3:2,4-dianhydride, cage-shape5-methyl-1,2,3,4-cyclopentanetetracarboxylic acid-1,3:2,4-dianhydride,and cage-shaped 5,5-dimethyl-1,2,3,4-cyclopentanetetracarboxylicacid-1,3:2,4-dianhydride.

According to the present invention, the polyamic acid contains therepeating unit represented by the formula [7] in an amount more than 10mol %. However, the content of the repeating unit should be more than 50mol %, preferably more than 80 mol %, or 100 mol %, in order for theresulting polyimide to have high transparency and good solubility inorganic solvents as intended in the present invention.

The diamine for polycondensation is not specifically restricted. Anyones conventionally used for polyimide synthesis are acceptable. Theirtypical examples are listed below.

Aromatic diamines, such as p-phenylenediamine, m-phenylenediamine,2,5-diaminotoluene, 2,6-diaminotoluene,1,3-bis(4,4′-aminophenoxy)benzene, 4,4′-diamino-1,5-phenoxypentane,4,4′-diaminobiphenyl, 3,3′-dimethyl-4,4′-diaminobiphenyl,3,3′-dimethoxy-4,4′-diaminobiphenyl, 4,4′-diaminodiphenyl ether,4,4′-diaminophenylmethane, 2,2′-diaminodiphenylpropane,bis(3,5-diethyl-4-aminophenyl)methane, diaminodiphenylsulfone,diaminobenzophenone, diaminonaphthalene, 1,4-bis(4-aminophenoxy)benzene,1,4-bis(4-aminophenyl)benzene, 9,10-bis(4-aminophenyl)anthracene,1,3-bis(4-aminophenoxy)benzene, 4,4′-bis(4-aminophenoxy)diphenylsulfone,2,2-bis[4-(4-aminophenoxy)phenyl]propane, and2,2′-trifluoromethyl-4,4′-diaminobiphenyl.

Alicyclic diamines, such as 1,4-diaminocyclohexane,1,4-cyclohexanebis(methylamine), 4,4′-diaminodicyclohexylmethane,bis(4-amino-3-methylcyclohexyl)methane,3(4),8(9)-bis(aminomethyl)tricyclo[5.2.1.0^(2,6)]decane,2,5(6)-bis(aminomethyl)bicyclo[2.2.1]heptane, 1,3-diaminoadamantane,3,3′-diamino-1,1′-biadamantyl, and1,6-diaminoadamantane(1,6-aminopentanecyclo-[7.3.1.1^(4,12),0^(2,7),0^(6,11)]tetradecane.

Aliphatic diamines, such as tetramethylenediamine andhexamethylenediamine.

Incidentally, these diamines may be used alone or in combination withone another.

Of these diamines, alicyclic and aliphatic ones are desirable becausethe resulting polyimide obtained via the polyamic acid excels intransparency.

The polyamic acid according to the present invention should have thediamine residue B whose preferable examples are represented by theformulas [9] to [12] below, where R⁴ to R¹¹ independently denote ahydrogen atom, halogen atom, or C₁₋₁₀ alkyl group, with a hydrogen atombeing desirable.

Incidentally, in the formula [10], m denotes an integer of 1 to 10,preferably 1 to 5. Therefore, the diamine residue B represented by anyone of the formulas [13] to [16] below is more desirable.

As mentioned above, the present invention requires that thetetracarboxylic acid dianhydride should contain the “cage” CPDArepresented by the formula [1] in an amount of at least 10 mol %.However, it may be used in combination with a tetracarboxylic acid or aderivative thereof which is used for ordinary polyimide synthesis, solong as the content of the “cage” CPDA is higher than 10 mol %.

Typical examples of the tetracarboxylic acid include alicyclictetracarboxylic acids, dianhydrides thereof, and dicarboxylic aciddiacid halides thereof, such as 1,2,3,4-cyclobutanetetracarboxylic acid,2,3,4,5-tetrahydrofurantetracarboxylic acid,1,2,4,5-cyclohexanetetracarboxylic acid,3,4-dicarboxy-1-cyclohexylsuccinic acid,3,4-dicarboxy-1,2,3,4-tetrahydro-1-naphthalenesuccinic acid, andbicyclo[3.3.0]octane-2,4,6,8-tetracarboxylic acid.

Additional examples include aromatic tetracarboxylic acids, dianhydridesthereof, and dicarboxylic acid diacid halides thereof, such aspyromellitic acid, 2,3,6,7-naphthalenetetracarboxylic acid,1,2,5,6-naphthalenetetracraboxylic acid,1,4,5,8-naphthalenetetracraboxylic acid,2,3,6,7-anthracenetetracarboxylic acid,1,2,5,6-anthracenetetracarboxylic acid,3,3′,4,4′-biphenyltetracarboxylic acid, 2,3,3′,4-biphenyltetracarboxylicacid, bis(3,4-dicarboxyphenyl)methane,3,3′,4,4′-benzophenonetetracarboxylic acid,bis(3,4-dicarboxyphenyl)methane, 2,2-bis(3,4-dicarboxyphenyl)propane,1,1,1,3,3,3-hexafluoro-2,2-bis(3,4-dicarboxyphenyl)propane,bis(3,4-dicarboxyphenyl)dimethylsilane,bis(3,4-dicarboxyphenyl)diphenylsilane, 2,3,4,5-pyridinetetracarboxylicacid, and 2,6-bis(3,4-dicaroxyphenyl)pyridine. Incidentally, theforegoing tetracarboxylic acids may be used alone or in combination withone another.

The polyamic acid according to the present invention may be obtained inany way without specific restrictions. One known way is by reaction of atetracarboxylic dianhydride and/or a derivative thereof with a diaminefor polymerization. One simple way is by reaction between atetracarboxylic dianhydride and a diamine which are mixed in an organicsolvent.

The organic solvent used for this purpose includes, for example,m-cresol, N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF),N,N-dimethylacetamide (DMAc), N-methylcaprolactam, dimethylsufoxide(DMSO), tetramethylurea, pyridine, dimethylsulfone,hexamethylphosphoramide, and γ-butyrolactone. These solvents may be usedalone or in combination with one another. Incidentally, they may be usedin combination with any solvent incapable of dissolving the polyamicacid so long as the mixed solvent gives a homogenous solution.

The solution polymerization may be carried out at any arbitrarytemperature ranging from −20° C. to 150° C., preferably from −5° C. to100° C. The resulting polyamic acid varies in molecular weight dependingon the molar ratio of the tetracarboxylic dianhydride and the diamineinvolved in polymerization. Its molecular weight increases according asthe molar ratio approaches one, as in the case of ordinarypolycondensation. The molar ratio of total tetracarboxylic dianhydridesto total diamines should preferably be from 0.8 to 1.2.

There are several methods for dissolving the tetracarboxylic dianhydrideand the diamine in an organic solvent. One method involves dispersion ordissolution of the diamine in an organic solvent and addition of thetetracarboxylic dianhydride (as such or dissolved or dispersed in anorganic solvent) to the solution of the diamine. In another method, theforegoing steps are carried out in a reverse order or alternately. Anyone of them may be employed in the present invention.

In the case where more than one species of tetracarboxylic dianhydrideor diamine are used, they may be put into reaction individually andsequentially or all together in the form of previously prepared mixture.

The polyamic acid obtained as mentioned above subsequently undergoesdehydrocyclization to give the polyimide of the present invention. Theratio of conversion from polyamic acid to polyimide is defined as theimidizing ratio. The present invention does not necessarily require theimidizing ratio to be 100% but permits it to vary from 1% to 100%.

The present invention does not restrict the method fordehydrocyclization of the polyamic acid. Heating or catalyst may beemployed as in the case of ordinary polyamic acid.

Dehydrocyclization by heating may be accomplished at any temperaturefrom 100° C. to 300° C., preferably from 120° C. to 250° C.

Dehydrocyclization by catalyst may be accomplished in the presence of anorganic base (such as pyridine and triethylamine) and acetic anhydrideat any temperature from −20° C. to 200° C. The resulting polyamicpolymer solution may be used as such or after dilution. An alternativemethod involves recovery of polyamic acid from the polyamic polymersolution and dissolution in an adequate organic solvent. The organicsolvent for this purpose is the same one as mentioned above which isused for polymerization of polyamic acid.

The thus obtained solution of (or containing) polyimide may be used assuch or undergo the subsequent step, in which it is given a poor solvent(such as methanol and ethanol) for precipitation of polymer and theprecipitates are separated. The separated precipitates (in powder form)may be used as such or after redissolution in an adequate solvent.

The solvent for redissolution is not specifically restricted so long asit is capable of dissolving the resulting polymer. It includes, forexample, m-cresol, 2-pyrrolidone, NMP, N-ethyl-2-pyrrolidone,N-vinyl-2-pyrrolidone, DMAc, DMF, and γ-butyrolactone.

Any solvent which does not dissolve the polymer when used alone may beused in combination with the foregoing solvents within an amount notharmful to solubility. Examples of such solvents are listed below.

Ethylcellosolve, butylcellosolve, ethylcarbitol, butylcarbitol,ethylcarbitol acetate, ethylene glycol, 1-methoxy-2-propanol,1-ethoxy-2-propanol, 1-butoxy-2-propanol, 1-phenoxy-2-propanol,propylene glycol monoacetate, propylene glycol diacetate, propyleneglycol-1-monomethy ether-2-acetate, propylene glycol-1-monoethylether-2-acetate, dipropylene glycol, 2-(2-ethoxypropoxy)propanol, methyllactate, ethyl lactate, n-propyl lactate, n-butyl lactate, and isoamyllactate.

The polyamic acid or polyimide according to the present invention is notspecifically restricted in molecular weight. Their molecular weightshould be properly selected according to their usage. With anexcessively small molecular weight, they will yield a material poor instrength. With an excessively large molecular weight, they will give asolution poor in workability.

They should have a number-average molecular weight of 2,000 to 500,000,preferably 5,000 to 300,000. This requirement is met by properlyselecting “n” (integer) in the formula [7] given above.

EXAMPLES

The invention will be described below in more detail with reference toExamples and Comparative Examples, which are not intended to restrictthe scope thereof. They involve measurement of characteristic propertieswith the following apparatus.

[1] ¹H-NMR

-   -   Model: Varian NMR System 400NB (400 MHz) ECP500 (JEOL)    -   Solvents: CDCl₃ and DMSO-d₆        [2] Mass spectrometry    -   Model: LX-1000 (JEOL)        [3] Melting point (m.p.)    -   Model: Micro melting point apparatus (MP-S3) (from Yanaco Kiki        Kaihatsu Kenkyusho)        [4] X-ray single crystal analysis    -   Model: M18XHF/DIP2030 (from Mac Science)    -   X-ray: MoK_(α) (45 kV, 200 mA)    -   Measured at room temperature        [5] Molecular weight of polyamic acid or polyimide    -   Model: Normal temperature gel permeation chromatograph (GPC)        (SSC-7200 from Senshu Kagaku), with Shodex's columns KD803 and        805, and DMF as eluent. The number-average and weight-average        molecular weights were obtained from calibration curves for        polyethylene glycol and polyethylene oxide as the standard        reference materials. The resulting polyimide was analyzed by        ¹H-NMR after dissolution in DMSO-d₆ and was examined for the        ratio of imidization by calculations from the ratio of the peak        area of protons assigned to benzene rings to the peak area of        protons assigned to amide residues remaining without being        imidized.

Example 1 Synthesis of cis,cis,cis-tetramethyl1,2,3,4-cyclopentane-tetracarboxylate (cis,cis,cis-TMCP)

There were placed 17.9 g (85 mmol) of cis,cis,cis-CPDA, 1.79 g of 95%sulfuric acid, and 89.5 g of methanol in a 200-mL four-neck flask ofPyrex (registered trademark) glass. The reactants were heated in an oilbath at 80° C. under reflux for 6 hours. After the completion ofreaction, the reaction product was condensed to give an oily substance(29 g). The oily substance was dissolved in ethyl acetate and water, andthe organic layer was separated, followed by water washing,concentration, and vacuum drying. Thus there was obtained 21.7 g ofcolorless oily substance (yields: 84.4%). This oily substance solidifiedat 25° C.

The resulting crystal was found to be cis,cis,cis-TMCP by ¹H-NMRanalysis.

¹H-NMR (CDCl₃, δ ppm): 2.398-2.453 (m, 1H), 2.779-2.838 (m, 1H),3.102-3.127 (m, 2H), 3.404-3.426 (m, 2H), 3.678-3.728 (m, 12H).

Example 2 Synthesis of trans,trans,trans-tetramethyl1,2,3,4-cyclopentanetetracarboxylate (trans,trans,trans-TMCP)

There were placed 9.06 g (30 mmol) of cis,cis,cis-TMCP, 1.01 g (30 mol%) of potassium t-butoxide (95% purity), and 63 g of methanol in a100-mL four-neck flask of Pyrex (registered trade mark) glass. Thereactants were heated in an oil bath at 80° C. under reflux for 2 hours.After the completion of reaction, the reaction product was concentratedand the resulting residue was dissolved in ethyl acetate and water. Theresulting solution was ice-cooled and acidified with 35% hydrochloricacid. The organic layer was separated and concentrated to give 8.48 g ofcrude oily substance (yields: 93.6%). This crude oily substance waspurified by silica gel chromatography (eluent: a mixture of ethylacetate and heptane, from 1/3 to 1/1). Thus there was obtained 6.14 ofoily substance (yields: 67.7%).

This oily substance was found to be trans,trans,trans-TMCP by ¹H-NMRanalysis.

¹H-NMR (CDCl₃, δ ppm): 1.220-1.296 (m, 6H), 2.308 (t, J=8 Hz, 2H),3.208-3.572 (m, 3H), 3.670-3.747 (m. 3H), 4.105-4.229 (m, 4H).

Example 3 Synthesis oftrans,trans,trans-1,2,3,4-cyclopentanetetracarboxylic acid(trans,trans,trans-CPTC)

There were placed 4.74 g (15.6 mmol) of trans,trans,trans-TMCP, 0.474 g(10 wt %) of p-toluenesulfonic acid monohydrate, and 33.2 g of formicacid in a 100-mL four-neck flask of Pyrex (registered trademark) glass.The reactants were heated at 130° C. under reflux for 7 hours, withmethyl formate (by-product) continuously distilled away. After thecompletion of reaction, the reaction product was concentrated to give3.60 g of fatty substance. This fatty substance was dissolved withheating in acetonitrile. The resulting solution was concentrated andice-cooled overnight for precipitation. The resulting white crystalswere filtered off and dissolved in a mixture of ethyl acetate andn-heptane (1/1 by volume), followed by washing and vacuum drying. Thusthere was obtained 2.30 g of white crystals (yield: 59.9%).

This substance was found to be trans,trans,trans-CPTC by massspectrometry and ¹H-NMR analysis.

MASS (ESI⁻, m/e(%)): 245 ([M-H]⁻, 100), 227 (57), 183 (18).

¹H-NMR (DMSO-d₆, δ ppm): 2.033-2.078 (m, 2H), 2.915-2.975 (m, 2H),3.178-3.195 m, 2H), 12.509 (brs, 4H).

m.p.: 208 to 209° C.

Example 4 Synthesis of “cage” CPDA

There were placed 7.80 g (31.7 mmol) of trans,trans,trans-CPTC and 26.0g (255 mmol) of acetic anhydride in a 100-mL four-neck flask of Pyrex(registered trademark) glass. The reactants were heated with stirring ata bath temperature of 110° C. for 7 minutes to give a uniform solution.After continued stirring for 10 minutes to complete the reaction, thereaction product was concentrated and the residue was dissolved in DMFwith heating. The solution was concentrated to 12.5 g to give a slurrycontaining crystals. The slurry was given ethyl acetate so that thetotal amount became 23.4 g. The resulting solution was heated andice-cooled overnight for precipitation. The precipitates (whitecrystals) were filtered off, washed with ethyl acetate, and vacuumdried. Thus there was obtained 5.13 g of white crystals (yield: 77.0%).

These crystals were found to be “cage” CPDA by ¹H-NMR analysis and X-raystructure analysis.

¹H-NMR (DMSO-d₆, δ ppm): 2.565-2.588 (m, 2H), 3.618-3.646 (m, 2H), 4.239(t, J=1.2 Hz, 2H)

m.p.: 228 to 230° C.

Results of X-Ray Analysis of “Cage” CPDA Single Crystal

The white crystals of “cage” CPDA obtained as mentioned above were usedas such for X-ray analysis of single crystal. The resulting ORTEPdiagram is shown in FIG. 1.

Crystallographic parameters

Molecular formula: C₉H₆O₆

Molecular weight: 210.14

Color, shape: colorless, plate

Crystal system: triclinic

Space group: P-1

Crystal form: plane

Lattice constant: a=6.621 (1) Å, b=11.007 (2) Å, c=12.191 (2) Å,

-   -   α=79.554°, β=89.969°, γ=72.474°

V=831.8 (2) Å³

Z value=4

D calc=1.513 Mg/m³

Mo K<α> radiation

λ (MoKa)=0.71072 Å

No. of measured reflections=950

No. of observed reflections=885

R=0.06

wR=0.08

Temp.=297K

Example 5 Synthesis of “Cage” CPDA-1,3-BAPB Polyamic Acid and Polyimide

There were placed 1.36 g (4.85 mmol) of 1,3-BAPB and 9.60 g of NMP in a50-mL four-neck flask (equipped with a stirrer) immersed in a water bathat 20° C. The reactant was dissolved by stirring at 185 rpm. Withstirring continued, the solution was given 1.02 g (4.85 mmol) of “cage”CPDA in small portions. Stirring was continued at 20 to 17° C., for 24hours for polymerization. Thus there was obtained a solution of polyamicacid, with a solid content of 20 wt %. This solution was found to have aviscosity of 332 mPa·s. The results of GPC analysis indicate that thepolyamic acid has a number-average molecular weight (Mn) of 11,178 and aweight-average molecular weight (Mw) of 23,424, with Mw/Mn being 2.10.

Subsequently, the solution was diluted with NMP (28 g) so that its solidcontent decreased to 6 wt %. The diluted solution was given 10.2 g (100mmol) of acetic anhydride, followed by stirring for 5 minutes. Theresulting solution was further given 4.75 g (60 mmol) of pyridine,followed by stirring at 100° C. for 5 hours.

After cooling to room temperature, the resulting solution was addeddropwise to 200 mL of methanol with stirring. Stirring was continued for1 hour for precipitation. The precipitated grayish powder was filteredoff and washed with 100 mL of methanol three times and finallyvacuum-dried at 80° C. for 2 hours. Thus there was obtained 2.0 g of“cage” CPDA-1,3-BAPB polyimide (yields: 88.3%). The ratio of imidizationwas 79.8% according to the data of ¹H-NMR analysis.

m.p.: 265 to 270° C.

Example 6 Synthesis of “Cage” CPDA-1,3-BAPB Polyamic Acid and Polyimide

There were placed 1.36 g (4.85 mmol) of 1,3-BAPB and 9.60 g of NMP in a50-mL four-neck flask (equipped with a stirrer) immersed in a water bathat 20° C. The reactant was dissolved by stirring at 185 rpm. Withstirring continued, the solution was given 1.02 g (4.85 mmol) of “cage”CPDA in small portions. Stirring was continued at 50° C., for 24 hoursfor polymerization. Thus there was obtained a solution of polyamic acid,with a solid content of 20 wt %. This solution was found to have aviscosity of 210 mPa·s. The results of GPC analysis indicate that thepolyamic acid has a number-average molecular weight (Mn) of 8,695 and aweight-average molecular weight (Mw) of 16,603, with Mw/Mn being 1.91.

Subsequently, the solution was diluted with NMP (28 g) so that its solidcontent decreased to 6 wt %. The diluted solution was given 10.2 g (100mmol) of acetic anhydride, followed by stirring for 5 minutes. Theresulting solution was further given 4.75 g (60 mmol) of pyridine,followed by stirring at 110° C. for 5 hours.

After cooling to room temperature, the resulting solution was addeddropwise to 160 mL of methanol with stirring. Stirring was continued for1 hour for precipitation. The precipitated grayish powder was filteredoff and washed with 120 mL of methanol three times and finallyvacuum-dried at 80° C. for 2 hours. Thus there was obtained 2.0 g of“cage” CPDA-1,3-BAPB polyimide (yields: 88.3%). The ratio of imidizationwas 77.9% according to the data of ¹H-NMR analysis. The results of GPCanalysis indicate that the polyimide has a number-average molecularweight (Mn) of 8,233 and a weight-average molecular weight (Mw) of15,067, with Mw/Mn being 1.83.

m.p.: 268 to 270° C.

Example 7 Synthesis of “Cage” CPDA-DPP Polyamic Acid and Polyimide

There were placed 1.39 g (4.85 mmol) of 4,4′-diamino-1,5-phenoxypentane(DPP) and 12.2 g of NMP in a 50-mL four-neck flask (equipped with astirrer) immersed in a water bath at 20° C. The reactant was dissolvedby stirring at 185 rpm. With stirring continued, the solution was given1.02 g (4.85 mmol) of “cage” CPDA in small portions. Stirring wascontinued at 20 to 17° C., for 24 hours for polymerization. Thus therewas obtained a solution of polyamic acid, with a solid content of 20 wt%. The viscosity of this solution was 5,920 mPa·s.

Subsequently, the solution was diluted with NMP (37.4 g) so that itssolid content decreased to 6 wt %. The diluted solution was given 10.2 g(100 mmol) of acetic anhydride, followed by stirring for 5 minutes. Theresulting solution was further given 4.75 g (60 mmol) of pyridine,followed by stirring at 100° C. for 2 hours.

After cooling to room temperature, the resulting solution was addeddropwise to 190 mL of methanol with stirring. Stirring was continued for1 hour for precipitation. The precipitated grayish powder was filteredoff and washed with 60 mL of methanol three times and finallyvacuum-dried at 80° C. for 2 hours. Thus there was obtained 2.27 g of“cage” CPDA-DDP polyimide in the form of violet granules (yields:98.5%). The ratio of imidization was 95.9% according to the data of¹H-NMR analysis. The results of GPC analysis indicate that the polyimidehas a number-average molecular weight (Mn) of 23,563 and aweight-average molecular weight (Mw) of 77,031, with Mw/Mn being 3.27.

m.p.: 275 to 280° C.

Example 8 Synthesis of “Cage” CPDA-p-PDA Polyamic Acid and Polyimide

There were placed 0.541 g (5.00 mmol) of p-phenylenediamine and 8.2 g ofNMP in a 50-mL four-neck flask (equipped with a stirrer) immersed in awater bath at 20° C. The reactant was dissolved by stirring at 185 rpm.With stirring continued, the solution was given 1.05 g (5.00 mmol) of“cage” CPDA in small portions. Stirring was continued at 20 to 17° C.,for 24 hours for polymerization. Thus there was obtained a solution ofpolyamic acid, with a solid content of 20 wt %. This solution was foundto have a viscosity of 1,355 mPa·s. The results of GPC analysis indicatethat the polyamic acid has a number-average molecular weight (Mn) of28,448 and a weight-average molecular weight (Mw) of 95,779, with Mw/Mnbeing 3.37.

Subsequently, the solution was given 10.2 g (100 mmol) of aceticanhydride, followed by stirring for 5 minutes. The resulting solutionwas further given 4.75 g (60 mmol) of pyridine, followed by stirring at110° C. for 2 hours. The solution yielded a gel-like substance.

After cooling to room temperature, the resulting solution was addeddropwise to 170 mL of methanol with stirring. Stirring was continued for1 hour to crush the gel-like substance. The precipitated violet gel-likesubstance was filtered off and washed with 90 mL of methanol three timesand finally vacuum-dried at 80° C. for 2 hours. Thus there was obtained1.48 g of “cage” CPDA-p-PDA polyimide (yields: 100%). The ratio ofimidization was 93.8% according to the data of ¹H-NMR analysis.

m.p.: >300° C.

Example 9 Synthesis of “Cage” CPDA-DDE Polyamic Acid and Polyimide

There were placed 1.00 g (5.00 mmol) of 4,4′-diaminodiphenyl ether (DDE)and 8.3 g of NMP in a 50-mL four-neck flask (equipped with a stirrer)immersed in a water bath at 20° C. The reactant was dissolved bystirring at 185 rpm. With stirring continued, the solution was given1.05 g (5.00 mmol) of “cage” CPDA in small portions. Stirring wascontinued at 20 to 17° C., for 15 hours. The solution turned into asyrupy viscous solution which wound around the stirrer shaft. Afterstirring at 50° C. for 2 hours with additional NMP (36.8 g), there wasobtained a polyamic acid solution with a solid content of 6 wt %. Thissolution was found to have a viscosity of 119 mPa·s. The results of GPCanalysis indicate that the polyamic acid has a number-average molecularweight (Mn) of 50,129 and a weight-average molecular weight (Mw) of241,300, with Mw/Mn being 4.81.

Subsequently, the solution was given 10.2 g (100 mmol) of aceticanhydride, followed by stirring for 5 minutes. The resulting solutionwas further given 4.75 g (60 mmol) of pyridine, followed by stirring at100° C. for 2 hours. The solution turned into a gel. The gel was madeinto a uniform solution by heating at 140° C. for 3 hours.

After cooling to room temperature, the resulting solution was addeddropwise to 220 mL of methanol with stirring. Stirring was continued for1 hour for precipitation. The precipitated grayish powder was filteredoff and washed with 90 mL of methanol three times and finallyvacuum-dried at 80° C. for 2 hours. Thus there was obtained 1.93 g of“cage” CPDA-DDE polyimide (yields: 100%). The ratio of imidization was95.8% according to the data of ¹H-NMR analysis.

m.p.: >300° C.

Comparative Example 1 Synthesis of CPDA-1,3-BAPB Polyamic Acid andPolyimide

There were placed 2.79 g (10.00 mmol) of 1,3-BAPB and 19.6 g of NMP in a50-mL four-neck flask (equipped with a stirrer) immersed in a water bathat 25° C., so that 1,3-BAPB was dissolved in NMP. With stirringcontinued, the solution was given 2.10 g (10 mmol) of1,2,3,4-cyclopentanetetracarboxylic acid-1,2:3,4-dianhydride (CPDA) insmall portions. The solution was stirred at 25° C. for 24 hours forpolymerization. There was obtained a polyamic acid solution with a solidcontent of 20 wt %. This solution was diluted with DMAc so that thesolid content decreased to 8 wt %. The diluted solution was stirred for5 hours with 10.2 g (100 mmol) of acetic anhydride and further stirredat 100° C. for 2 hours with 7.9 g (100 mmol) of pyridine.

After cooling to room temperature, the resulting solution was addeddropwise to 3.5 times as much (in volume) water as the solution withstirring. Stirring was continued for 30 minutes for precipitation. Theprecipitated white powder was filtered off and washed with water andvacuum-dried at 80° C. for 2 hours. Thus there was obtained 3.8 g ofCPDA-1,3-BAPB polyimide (yields: 83.8%). The ratio of imidization was90.1% according to the data of ¹H-NMR analysis. The results of GPCanalysis indicate that the polyamic acid has a number-average molecularweight (Mn) of 2,421 and a weight-average molecular weight (Mw) of3,030, with Mw/Mn being 1.25.

m.p.: 193 to 195° C.

Examples and Comparative Example mentioned above show that the polyimideaccording to the present invention has a higher molecular weight thanCPDA polyimide. They also show that the former has a higher meltingpoint (above 260° C.) than the latter (below 200° C.). This implies thatthe former is superior to the latter in heat resistance.

[Solubility of Polyimide]

The polyimide samples obtained in Examples 5 and 6 and ComparativeExample 1 were tested as follows for solubility in various organicsolvents shown in Table 1. The results are also shown in Table 1.

<Method for Solubility Test>

The solubility was rated according to the following criteria byobservation of each sample (2 mg) added to the organic solvent (0.2 mL)with stirring.

⊚: completely soluble at 25° C. (room temperature)

◯: completely soluble at 80° C. (with heating)

Δ: partly soluble at 80° C. (with heating)

x: insoluble at 80° C.

DMSO: dimethylsulfoxide, DMF: N,N-dimethylformamide, THF:tetrahydrofuran

TABLE 1 Solubility of “cage” CPDA-PI in Organic Solvents Organic solventExample 5 Example 6 *Comparative Example 1 DMSO ⊚ ⊚ ⊚ DMF ⊚ ⊚ ⊚γ-butyrolactone ⊚ ⊚ Δ m-cresol ◯ ⊚ Δ Pyridine ⊚ ⊚ ⊚ 1,4-dioxane Δ ⊚ XTHF X Δ X ⊚: completely soluble at room temperature ◯: completelysoluble with heating Δ: partly soluble with heating X: insoluble withheating

It is noted from Table 1 that the sample of “cage” CPDA-1,3-BAPBpolyimide obtained in Examples 5 and 6 are more soluble in organicsolvents than the sample of CPDA-1,3-BAPB polyimide despite their highernumber-average molecular weight (Mn) and weight-average molecular weight(Mw).

The invention claimed is:
 1. A polyamic acid which contains therepeating unit represented by the formula [7] below in an amount of atleast 10 mol %

wherein A denotes a tetravalent organic group represented by the formula[8] and B denotes a divalent organic group and n is an integer

wherein R¹ and R² independently denote a hydrogen atom, halogen atom, orC₁₋₁₀ alkyl group; and a1 to a4 denote the positions for bonding withthe carbon atom of the carbonyl group in the formula [7], provided thatbonding with the carboxyl group does not take place simultaneously at a1and a3 and bonding with the carboxyl group does not take placesimultaneously at a2 and a4.
 2. The polyamic acid as defined in claim 1wherein said R¹ and R² each is a hydrogen atom or methyl group.
 3. Thepolyamic acid as defined in claim 1 wherein said B is a divalent organicgroup derived from an alicyclic diamine or aliphatic diamine.
 4. Thepolyamic acid as defined in claim 1 or 2, wherein said B is at least onespecies selected from the divalent organic groups represented by theformulas [9] to [12]

wherein R⁴ to R¹¹ independently denote a hydrogen atom, halogen atom, orC₁₋₁₀ alkyl group, and m is an integer of 1 to
 10. 5. The polyamic acidas defined in claim 4, wherein said B is represented by the formula [13]


6. The polyamic acid as defined in claim 4, wherein said B isrepresented by the formula [14]


7. The polyamic acid as defined in claim 4, wherein said B isrepresented by the formula [15]


8. The polyamic acid as defined in claim 4, wherein said B isrepresented by the formula [16]